Log periodic backward wave antenna array



Oct. 22, 1963 Filed Sept. 30, 1960 I Beam Dlrecflon Fig. 1

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P. E. MAYE S ETAL LOG PERIODIC BACKWARD WAVE ANTENNA ARRAY 2Sheets-Sheet 1 INVENTORS Paul E. Mayes Robert L. Carrel Merriam, Smith 8Marshall A 7' T OR/VE Y5 Oct. 22, 1963 P; E. MAYES ETAL 3,108,280

LOG PERIODIC BACKWARD WAVE ANTENNA ARRAY Filed Sept. 30, 1960 2Sheets-Sheet 2 N if,

INVENTORS Paul E. Mayes BY Ruben L. Carrel Merriam, Smith 8 Ma'rslmllATTOR/VE') United States Patent 3,108,280 L8G PERHGDIC BACKWARD WAVEANTENNA ARRAY Paul E. Mayes, Champaign, and Robert L. (Farrel,

Urbana, llit, assignors to The University of Illinois Foundation, anon-profit organization of Illinois Filed Sept. 30, 196i Ser. No. 59,671it) (liaims. (Ci. 343-7925) This invention relates to antennas and moreparticularly it relates to antennas having unidirectional radia tionpatterns that are essentially independent of frequency over widebandwidths.

In the copending application of Dwight E. Isbell, Ser. No. 26,589, filedMay 3, 1960-, there are described certain antennas comprising coplanardipole arrays which rave an unusually wide bandwidth over which theperformance of the antennas is essentially frequency independent and theinput impedance nearly constant, the antennas also having aunidirectional pattern with a directivity comparable to a Yagiarray. Asdescribed in the aforementioned application, these arrays comprise anumber of dipoles arranged in side-by-side relationship in a plane thelength of the dipoles and the spacing between adjacent dipoles varyingaccording to a definite mathematical formula, with each of the dipolesbeing fed at its midpoint by a common feeder which introduces an addedphase shift of 180 between connections to suecessive dipoles. Thedipoles which are used to make up the array vary progressively inlength, the longest dipole element being about wavelength long at thelow frequency limit of a given antennas effective range and the shortestelement being about /8 wavelength long at the upper frequency limit.

in accordance with the present invention, it has been found that thedirectivity of an antenna of the type described in the aforementionedapplication may be increased and the effective frequency range of anantenna of fixed size may be extended by inclining the dipoles of Isbellto form ll-elements, each of 'which consists of two straight arms ofequal length definingan apex which points away from the direction ofradiation of the antenna which is also the direction in which theelement size decreases. The modification of the straight dipoles ofIsbell to ll-shaped elements permits the antenna to be operated overbands of frequencies higher than those established, as described above,by the length of the shortest dipole in the antenna, with increaseddirectivity, thus obviously increasing the effective frequency range ofa given antenna.

The invention will be better understood from the following detaileddescription thereof taken in conjunction with the accompanying drawings,in which the same numbers are used to denote corresponding elements inthe several views and in which:

FIGURE 1 is a schematic plan View of an antenna made in accordance withthe principles of the invention;

FIGURE 2 is a perspective view of a practical antenna embodying theinvention; and

FIGURE 3 is a fragmentary view of an improved and preferred form of anantenna similar to that shown in v FIGURE 2, as seen from a pointdirectly in front of and above the narrow end of the antenna.

Referring to FIGURE 1, it will be seen that the antennas of theinvention are composed of a plurality of \l-elements, e.g., l1 and 12,each of which consists of a pair of arms, e.g., l3 and 14, defining anapex in the middle of the V-elements, said V-elements being arranged ina herringbonelike pattern. The arms of a given V-element are equal inlength and corresponding arms of the several V-elements, i.e., the armson the same side of a line passing through the apexes of the V-elements,are

substantially parallel to each other. It will be noted that the lengthsof the arms of successive V-elements and the spacing between the apexesof the elements are such that the extremities of the elements fall on apair of straight lines which intersect to form an angle a. In thepreferred embodiment of the invention the antenna is symmetrical about aline passing through the apexes of the V-elements, as shown.

The antenna is fed at its narrow end from a conventional sourceofenergy, depicted in FIGURE 1' by alternator 16, by means of a balancedfeeder line consisting of conductors 17 and 18. It will be seen that thefeeder lines 17 and 13 are alternated between connections to consecutiveV-elements, thereby producing a phase reversal between such connections.

The lengths of the arms in the antenna, and the spac ing between theV-elements, are related by a constant scale factor '1' defined by thefollowing equations:

where 7- is a constant having a value less than 1, I is the length of anarm in any intermediate V-element in the array, I a is the length of anarm in the adjacent smaller V-element, the subscript n designating thenth arm running in an order from larger to smaller, AS, is the spacingbetween the apex of the V-element having the arm length I and the apexof the adjacent larger V-element, and AS is the spacing between the apexof the V-element having the arm length I and the apex of the adjacentsmaller V-element.

The arms of the individual V-elements forming the antenna array areinclined to point in the direction of decreasing V-element size so thatthe apex of each of the elements points in a direction away from theangle a. formed by the lines passing through the extremities of theindividual V-elements.

The angle formed by the arms of a V-element is designated as t. It willbe seen that when the angle 31/ is equal to 180", the antennas of theinvention are identical with those described by Isbell in theapplication mentioned above. In the instant invention, howeventhe angleit preferably has a value between about 50 and It will be seen from thegeometry of the invention as given above that the distances from thebase line 0 at the vertex of the angle at to the apexes of theV-elements forming the array are defined by the equation:

where X is the distance from the base line 0 to the apex of theV-element having the arm length I X n+1 is the corresponding distancefrom the base line to the apex of the adjacent smaller V-element, the 7-has the significance previously given.

The radiation pattern of the antennas of the invention having thegeometrical relationship among the several parts, as defined above, isunidirectional [in the negative X direction, i.e., extending to the leftfrom the narrow end of the antenna of FIGURE 1.

The use of V-elements in the antennas of the invention, rather thandipoles, increases the directivtity of the invention and also permitsmore effective utilization of a given antenna since the same structurecan be used in several frequency modes to achieve coverage of differentfrequency bands. In the special case of an antenna having straightdipoles rather than V-elements (i.e., when =180), the effectivefrequency range is that in which the low limit corresponds to thatfrequency in which the largest dipole in the antenna is about A2wavelength long and the upper frequency limit to that frequency in whichthe smallest dipole in the antenna is about Wavelength art-sass long. Ingeneral, therefore, it may be said that the frequency range of thestraight dipole array corresponds to the mode of operation in which thelengths of the dipoles in the array are about /2 wavelength long. As thefrequency is raised above the upperlimit of the /2 Wavelength mode inthe dipole array, the antenna will also be found to radiate effectivelyat frequencies in which the dipoles are about wavelengths long (theWavelengths mode), wavelengths long (the wavelengths mode) and so on. Atfrequencies above the half-wavelength mode, however, the radiationpattern of the dipole array becomes multilobed and is, therefore, oflimited usefulness. By including the arms of the dipole to form theV-elements of the instant invention, it has been found that a singlelobe of improved directivity may be obtained as the frequency is raisedfrom the half-wavelength mode through the intervening ranges to thewavelengths rnode and beyond. For each mode of operation there exists anoptimum value for the angle 0, ranging from about 114 for thehalf-wavelength mode to about 62 for the Wavelengths mode. By using acompromise value for 1 within this range, however, a practical antennacan be made to achieve acceptable performance over several modes ofoperation, thereby increasing its effective range without increasing thenumber of elements therein. This result is possible since many of theelements forming the antenna array are used at more than one frequency.

The construction of an actual antenna made in accordance with theinvention is shown in FIGURE 2. In this antenna the balanced lineconsists of two closely-spaced and parallel electrically conductingsmall diameter tubes 21 and 22 which also act as a mechanical supportfor the dipole elements and to which are attached the arms which formthe V-elements of the invention. It will be noted that each of the twoarms, e.g., 23 and 24, making up one V-element is connected to adifferent one of said conductors 21 and 22. Moreover, considering eitherone of the conductors 21 and 22, consecutive arms along the lengththereof extend in opposite directions. It will be seen that thisconstruction has the effect of alternating the phase of the connectionsbetween successive V-elements, as depicted schematically in FIGURE 1.Although the V-elements of FIGURE 2 are not precisely coplanar,differing therefrom by the distance between the parallel conductors 21and 22, in practice this distance is usually small so that the arms ofthe V-elements are substantially coplanar and the advantages of theinvention are maintained. In some instances, however, it may beadvantageous to bend the individual arms, e.g., 27 and 28, close to thepoint of attachment to the feeder line, as shown in FIGURE 3, so as toposition all the arms in the same plane. The antennas of FIGURES 2 and 3may be conveniently fed by means of a coaxial cable 25 positioned withinconductor 21, the outer conductor of the cable making electrical contactwith conductor 21 and the central conductor 26 of the cable extending toand making electrical connection with conductor 22, as shown.

The antennas of the invention may also be fed by a balanced two wireline which is twisted between elements to achieve the desired phasereversal. Other methods of achieving the desired phasing may beemployed, e.g., transmission line loops or stubs.

As an example of the invention, an antenna of the type shown in FIGURE 3was constructed using 0.125" diameter tubing for the balanced line and0.050 diameter wire for the arms of the V-elements. The arms weresoldered to the feeder line and the array Was fed by a miniature coaxialcable inserted into one of the conductors of the balanced line. Theantenna had 25 arms, the largest of which was 1 ft. long with theshortest being about 3 /2" long. The antenna was further defined by theparameters 1:095 and 0:70". This antenna exhibited typical directivitygains ranging from 12 db over isotropic in the W wavelengths mode to 17db in the wavelengths mode, with essentially constant input impedancewithin each mode.

Except with respect to the angle of inclination of the arms of theV-elements, the parameters which define the antennas or" the inventionare essentially similar to those of the corresponding strai ht dipolearrays in which the arms extend at right angles from the feeder lines.Thus, the parameter 1- preferably has a value betwen about 0.8 and 0.95and the angle at suitably ranges between 20 and Moreover, the upper andlower limits of the bandwidth for the /2 wavelength mode of operationcan be adjusted as desired by making the longest V-cleinent correspondin length to about /2 wavelength at the lower limit and the shortestV-element to about wavelength at the upper frequency limit.

In addition to its use as a direct radiator or receiver, the resonant-Varray of the invention has several advantages over other antennascurrently used as primary feeds for parabolic and other reflectors. Itsindependence of frequency in any single mode assures constantillumination of the reflector. Moreover, the input impedance remainsessentially independent of frequency so that no tuning is required asthe frequency is varied.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. A broadband unidirectional antenna comprising an array of a pluralityof V-elements in a planar herringbonelike arrangement, each of saidelements having a pair of equal arms defining an apex, the apexes ofsaid V- elements lying on a straight line the corresponding arms of saidelements progressively increasing in length and spacing, the extremitiesof the arms of said V-elements substantially falling on a V-shaped lineforming an angle a at its vertex, the 'apexes of said V-elementspointing in a direction away from the vertex of said angle a, the ratioof the arm lengths of any pair of adjacent V-elernents being given bythe formula where I is the length of an arm in the larger of said pairof V-elements, l is the length of an arm in the adjacent smallerV-element of said pair, the subscript n designating the nth arm runningin an order from larger to smaller, and T is a constant having a valueless than 1, the spacing between the apexes of said V-elements beinggiven by the formula ASH where AS is the spacing between the V-elementhaving the arm length I and the adjacent larger V-element, AS is thespacing between the V-element having the arm length I and the adjacentsmaller V-element, and 1 has the significance previously assigned, saidV-elements being adapted to be fed as a group from the small end of theindividual V-elements fed at the apexes thereof by a common feeder whichintroduces an additional 180 phase shift between successive V-elements.

2. The antenna of claim 1 wherein the angle formed by the arms of anyV-element at the apex thereof has a value within the range from about 50to about 3. The antenna of claim 1 which is symmetrical about a linepassing through the apex of each V-element therein, and in which thecorresponding arms of the V-elements are parallel.

4. The antenna of claim 1 in which the angle a has a value between about20 and 100 and the constant 1- has a value between about 0.8 and 0.95.

5. A broadband unidirectional antenna comprising a balanced feeder lineconsisting of two closely spaced, straight and parallel conductors, aplurality of substantially coplanar V-elements, each V-elernentcomprising a pair of arms of equal length defining an apex, one of saidarms of each V-element being connected at the apex of said V-element toone of said conductors, the other of said arms being connected directlyopposite the first to the other of said conductors, the arms of anyV-element extending in opposite directions at an acute angle to theplane determined by said conductors, consecutive arms on each of saidconductors extending on opposite sides of said plane, the ratio of thelengths of the arms in adjacent V-elements being given by the formulaWhere AS is the spacing between the V-element having the arm length Iand the adjacent larger V-element, A8 is the spacing between theV-element having the arm length I and the adjacent smaller V-element,and 1- has the significance previously assigned.

6. The antenna of claim 5 in which the angle formed by said arms withthe plane determined by said feeder line, measured in a planeperpendicular to said plane, has a value between about 25 and about 75.

7. The antenna of claim 5 in which ahas a value of about 0.8 to 0.95

8. An aerial system for wideband use comprising a plurality ofherringbone-like conducting V-elements planarly arranged, atwo-conductor balanced feeder connected to each of said elements atsubstantially the inner end thereof, each two opposite V-elementsforming a pair constituting dipole halves, the connection from eachadjacent dipole section being to a different feeder, said if-elementsbeing selectively spaced from each other, each V-element of each pairhaving arms of substantially equal length substantially defining an apexwith the apexes of the plurality of V-elements all lying insubstantially a straight line and terminating at the feeder, the saiddipoles of each pair being of different electrical lengths withsuccessive dipoles differing in electrical length with respect to eachother by substantially the same scale factor, each dipole and the feederbetween successive dipoles constituting a cell, and the selectivespacings between adjacent dipoles decreasing from one end to the otherwith the greater spacing being between the longest dipoles and beingsuch that the combination of dipole lengths and spacings provides asubstantially uniform wideband response over a plurality of frequencybands bearing substantially harmonic frequency relationships to eachother, the connection between the dipoles and the feeder being made insuch a manner that the directive gain of the antenna increases asoperation shifts from one band to an adjacent band of higherfrequencies, and means to connect the feeder to an external circuit at alocation substantially removed from the longest of the V-elernents andin the direction of the smallest of the V- elements.

9. An aerial system for wide-band use including a twoconductor balancedfeeder extending in a selected plane, a plurality of herringbone-likeconducting V-elements planarly arranged and spaced along the feeder,each of the elements having a pair of arms of substantially equal lengthdefining substantially an apex with the apexes of the plurality ofV-elements all lying in substantially a straight line and allterminating at the feeder, a connection between each of the V-elementsand one of the feeders at the inner end of the elements, the twoV-elemcnts forming each pair constituting dipole halves, adjacent dipolesections being connected to different feeders, each of 5 the pairs ofdipoles being of different electrical lengths with successive dipolesdiffering in electrical length with respect to each other bysubstantially a common scale factor, each dipole and the feederconnected thereto in the region between one dipole pair and the nextadjacent dipole pair constituting a cell, the spacings between thedipoles as connected to the feeders differing from each other also by"substantially the same common scale factor, the scale factor being sochosen that the combination of dipole lengths and spacings providing theseveral cells have a substantially uniform wide-band response oversevral frequency bands bearing substantially harmonic frequencyrelationships to each other, the connection between the feeder and thedipoles being made in such a manner that the directive gain of theantenna increases with operational shift from one band to another bandof higher frequency, and means to connect the feeder to an externalcircuit at a location substantially removed from the longest of theV-elements in the direction of the smallest of the V-elements.

10. An aerial system for wide-band use including an elongatedtwo-conductor balanced feeder, a plurality of herringbone-likeconducting V-elements plananly arranged and spaced along said feeder,each of the elements having a pair of arms of equal length definingsubstantially an apex with the apexes of the plurality of V-elements alllying in a substantially straight line, a connection between each of theV-elements and the feeder to terminate the elements substantially at thefeeder, the two V-elements foaming each pair constituting dipole halves,adjacent dipole sections of the plurality being connected to differentfeeders and the dipoles being relatively spaced so that the spacingsbetween successive dipoles differ from each other by substantially acommon scale factor, adjacent dipole sections having differentelectrical lengths, each dipole and the feeder connected between it andthe adjacent dipole constituting a cell, the lengths of the dipolesincreasing from end of array where spacings between adjacent dipoles isless to end of the array where adjacent dipoles are spaced the greatestdistance, the spacings by 45 the scale factor variation between adjacentdipoles being such that a combination of the various dipole lengths andspacings provides a substantially uniform wide-band response overseveral frequency bands bearing substantially harmonic frequencyrelationships to each other, the 50 connection being made in such amanner that the directive gain of the antenna increases as the operationshifts from one band to another band of higher frequency, and means toconnect the feeder to an external circuit at a location substantiallyremoved from the longest of the V-elements in the direction of thesmallest of the V-elements.

References Cited in the tile of this patent UNITED STATES PATENTS 1960;vol. AP-8, No. 3, pages 2'60267.

Channel Master Corp, KO. Antenna, copyright 1955, 3 pages.

Patent No. 3,108,280 October 22, 1963 Paul E. Mayes et a1.

It is hereby certified that error 2.

ant requiring correetion and that the sa ppears in the above numberedpatcorrected below.

id Letters Patent should read as Column 3, line 13, for "including" readinclining Signed and sealed this 26th day of May 1964.

(SEAL) Attest:

ERNEST Wq SWIDER EDWARD J. BRENNER Attesting Officer Commissioner ofPatents

10. AN AERIAL SYSTEM FOR WIDE-BAND USE INCLUDING AN ELONGATEDTWO-CONDUCTOR BALANCED FEEDER, A PLURALITY OF HERRINGBONE-LIKECONDUCTING V-ELEMENTS PLANARLY ARRANGED AND SPACED ALONG SAID FEEDER,EACH OF THE ELEMENTS HAVING A PAIR OF ARMS OF EQUAL LENGTH DEFININGSUBSTANTIALLY AN APEX WITH THE APEXES OF THE PLURALITY OF V-ELEMENTS ALLLYING IN A SUBSTANTIALLY STRAIGHT LINE, A CONNECTION BETWEEN EACH OF THEV-ELEMENTS AND THE FEEDER TO TERMINATE THE ELEMENTS SUBSTANTIALLY AT THEFEEDER, THE TWO V-ELEMENTS FORMING EACH PAIR CONSTITUTING DIPOLE HALVES,ADJACENT DIPOLE SECTIONS OF THE PLURALITY BEING CONNECTED TO DIFFERENTFEEDERS AND THE DIPOLES BEING RELATIVELY SPACED SO THAT THE SPACINGSBETWEEN SUCCESSIVE DIPOLES DIFFER FROM EACH OTHER BY SUBSTANTIALLY ACOMMON SCALE FACTOR, ADJACENT DIPOLE SECTIONS HAVING DIFFERENTELECTRICAL LENGTHS, EACH DIPOLE AND THE FEEDER CONNECTED BETWEEN IT ANDTHE ADJACENT DIPOLE CONSTITUTING A CELL, THE LENGTHS OF THE DIPOLESINCREASING FROM END OF ARRAY WHERE SPACINGS BETWEEN ADJACENT DIPOLES ISLESS TO END OF THE ARRAY WHERE ADJACENT DIPOLES ARE SPACED THE GREATESTDISTANCE, THE SPACINGS BY THE SCALE FACTOR VARIATION BETWEEN ADJACENTDIPOLES BEING SUCH THAT A COMBINATION OF THE VARIOUS DIPOLES LENGTHS ANDSPACINGS PROVIDES A SUBSTANTIALLY UNIFORM WIDE-BAND RESPONSE OVERSEVERAL FREQUENCY BANDS BEARING SUBSTANTIALLY HARMONIC FREQUENCYRELATIONSHIPS TO EACH OTHER, THE CONNECTION BEING MADE IN SUCH A MANNERTHAT THE DIRECTIVE GAIN OF THE ANTENNA INCREASES AS THE OPERATION SHIFTSFROM ONE BAND TO ANOTHER BAND OF HIGHER FREQUECY, AND MEANS TO CONNECTTHE FEEDER TO AN EXTERNAL CIRCUIT AT A LOCATION SUBSTANTIALLY REMOVEDFROM THE LONGEST OF THE V-ELEMENTS IN THE DIRECTION OF THE SMALLEST OFTHE V-ELEMENTS.